U.S. Pat. No. 9,443,180 is assigned to the present assignee and incorporated herein by reference. That patent discloses details of methods to print microscopic LEDs on a security label or directly on an object to be authenticated, where the LEDs are naturally randomly arranged within the security mark, such as within a 1 cm2 area. For example, there may be 30-50 LEDs within the security mark, and each security mark is inherently different. In that patent, the optically detected specific arrangement of the LEDs constitutes the unique signature of the security mark.
It is desirable to expand on the general concept of using printed LEDs in an object to visually detect the emitted light for reasons including security and feedback.